This invention relates generally to carbonaceous materials that have enhanced properties. More particularly, the present invention is related to carbon material that is made oxidation resistant to temperatures of 900xc2x0 C. The oxidation resistant carbon materials have an electrically non-conducting surface with significantly enhanced surface hardeness.
Carbonaceous materials, such as carbon, graphite, carbon-carbon composites, glassy carbon, and the like have many uses. In particular they are useful at high-temperatures where they have excellent mechanical strength. The oxidation of carbonaceous materials in air or oxygen-containing environments at temperatures of 400 to 500xc2x0 C. has limited its use in high-temperature applications. Otherwise, the easy machinability, low density, good strength, and other properties would lead to carbonaceous materials being the obvious choice.
Oxidation protection of carbonaceous materials has been directed to coatings and layers that are utilized to reduce the reaction of oxygen with the materials. Exemplary teachings are provided in U.S. Pat. Nos. 4,711,666 and 4,769,074. Often such layers contain silicon or aluminum to help form glasslike coatings during oxygen attack, whereby the glassy layer or glaze will reduce any additional oxidation of the substrate. An inherent concern with coatings is the thermal expansion mismatches between the substrate and coating that often cause delamination and complete coating spallation.
Another example of oxidation improvement for carbonaceous materials is U.S. Pat. No. 5,368,938, wherein described is the reaction of carbon with gaseous boron oxide to form boron carbide. Still another method of oxidation protection for carbonaceous materials, described in U.S. Pat. No. 5,356,727, is based on xe2x80x9cboron carbonitridexe2x80x9d designated as CBN, or CBNO if it contains oxygen. CBN is produced by chemical vapor deposition at 700xc2x0 C. with a mixture of hydrocarbons, boron trichloride and ammonia along with nitrogen or hydrogen carriers at a low as a small fraction of atmospheric pressure, such as a few hundred to a few thousand pascals. The CBN, as described therein, typically has a xe2x80x9cmetallic appearancexe2x80x9d at 50 micrometers thickness.
Graphite has been coated with xe2x80x9cpyrolytic boron nitridexe2x80x9d to form boats for metal vaporization, as described in U.S. Pat. No. 4,264,803. In such cases, the boron nitride coating was deposited at 1750 to 2300xc2x0 C. to a thickness of about 250 micrometers or 0.010 inches. It was found that the geometry of the boat cavity and nearly total encapsulation of the boat held the coating onto the substrate. The tendency of the coating of xe2x80x9cpyrolytic boron nitridexe2x80x9d to delaminate seems to be the main problem with this type of boat.
None of the known technologies for improving the oxidation resistance of carbonaceous materials produces a carbon material that is not a coated surface. Integral materials have been heretofore been thought to be difficult to prepare due to the differences in crystal lattice between dissimilar materials. Any blending of materials would generate a unique crystalline lattice which is dissimilar from either starting material. This typically leads to crystallographic defects and dislocations which can create additional, often uncontrollable and unpredictable, crystallographic phases.
A particular feature of the present invention is the ability to form carbonaceous materials with a hardened exterior that is non-conducting.
Another feature is the ability to form a relatively soft carbonaceous item in a desired shape and configuration after which the item can be treated to form an oxidative resistant hard surface without altering the dimensions or structural components of the carbonaceous item.
These and other advantages, as would be realised to one of ordinary skill in the art, are provided in a carbon material produced by heating a carbonaceous material embedded in a boron nitride precursor.
Another embodiment is provided in a process for manufacturing a carbon material that has enhanced oxidation resistance and an electrically non-conducting surface. The process involves the steps of embedding a carbonaceous material in a boron nitride precursor and heating the embedded carbonaceous material to a temperature in the range of from about 1600xc2x0 C. to about 2000xc2x0 C. at one atmosphere pressure with flowing nitrogen.
Yet another embodiment is provided in a surface hardened carbonaceous tool prepared by a process comprising machining a carbonaceous blank into a tool precursor, embedding the tool precursor in a boron nitride precursor to form an envelope and heating the envelope to a temperature of 1600 to 2000xc2x0 C. at one atmosphere of flowing nitrogen to form the carbonaceous tool.